Ceramic composite materials and methods

ABSTRACT

Provided herein are methods of making composite materials. The methods may include infiltrating a carbon nanoscale fiber network with a ceramic precursor, curing the ceramic precursor, and/or pyrolyzing the ceramic precursor. The infiltrating, curing, and pyrolyzing steps may be repeated one or more times. Composite materials also are provided that include a ceramic material and carbon nanoscale fibers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/325,748, filed Apr. 21, 2016, which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numbersN00014-11-1-0706 and N00014-14-1-0543 awarded by the Office of NavalResearch. The government has certain rights in the invention.

BACKGROUND

Ceramic composites, which may be used in a number of applications,typically include a ductile phase embedded in a ceramic material.

Existing methods of making ceramic composites typically includeembedding ceramic powders in a polymer phase. Due to the presence of apolymer phase, the flexibility of these ceramic composites is generallyrestricted to mild temperatures (e.g., 200° C. or less). Moreover,strong ionic/covalent bonding typically imparts conventional ceramiccomposites with strong mechanical properties while undermining thecomposites' flexibility.

These limitations can be disadvantageous, because many applications maybenefit from or require compliant and/or flexible ceramic composites.The ability to deform flexibly is desirable for many advancedapplications, such as thermal protection systems and battery materials.

Carbon nanotubes (CNTs) are known for their remarkable intrinsicmechanical, electrical, and thermal properties. As a result, CNTs havebeen used to improve the electrical properties of composite materials,but the CNTs can be difficult to disperse, completely or otherwise, intomatrices, including ceramic matrices. Existing methods for making acomposite containing ceramic materials and carbon nanotubes are based onmixing carbon nanotubes in ceramic powders or ceramic polymericprecursors directly. These methods, however, can produce compositescontaining only a limited volume fraction of carbon nanotubes in theceramic matrix.

There remains a need for improved composite materials and methods ofmaking composite materials containing carbon nanotubes and a ceramicmatrix, including composite materials that include a relatively highvolume fraction of carbon nanotubes, are flexible, and/or have arelatively high electrical conductivity.

BRIEF SUMMARY

Provided herein are composite materials that address one or more of theforegoing needs, and include a ceramic material and carbon nanoscalefibers.

In one aspect, methods of forming composite materials are provided. Themethods, in embodiments, comprise [1] providing a carbon nanoscale fibernetwork which comprises a plurality of substantially aligned carbonnanoscale fibers; [2] infiltrating the carbon nanoscale fiber networkwith a first amount of a liquid ceramic precursor; [3] curing the firstamount of the liquid ceramic precursor to form a cured ceramicprecursor; and [4] pyrolyzing the cured ceramic precursor to form thecomposite material. The volume fraction of the carbon nanoscale fibersin the composite materials may be at least 35%, or about 40% to about80%. The electrical conductivity of the composite material may be about2.0×10⁴ S/m to about 3.0×10⁴ S/m. In particular embodiments, the methodsfurther comprise [1] infiltrating a second amount of the liquid ceramicprecursor into the composite material; [2] curing the second amount ofthe liquid ceramic precursor to form a second amount of a cured ceramicprecursor; and [3] pyrolyzing the second amount of the cured ceramicprecursor.

In another aspect, composite materials are provided. In embodiments, thecomposite materials comprise a ceramic material dispersed in a carbonnanoscale fiber network which comprises a plurality of substantiallyaligned carbon nanoscale fibers. The composite material may have (i) avolume fraction of the carbon nanoscale fibers of at least 35%, and/or(ii) an electrical conductivity of at least 1.75×10⁴ S/m. The compositematerials also may be flexible, as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the shrinkage of one embodiment of a composite materialupon pyrolysis.

FIG. 2A depicts the stress-strain curves of one embodiment of acomposite material prior to and after several flexibility tests.

FIG. 2B depicts the Young's modulus of one embodiment of a compositematerial prior to and after several flexibility tests.

FIG. 3A depicts the stress-strain curves of two embodiments of acomposite material, an embodiment of a composite material prior topyrolysis, and an embodiment of an aligned CNT sheet.

FIG. 3B depicts the Young's modulus and mass change of two embodimentsof a composite material, an embodiment of a composite material prior topyrolysis, and an embodiment of an aligned CNT sheet.

FIG. 4 depicts the electrical resistivity in two directions of twoembodiments of a composite material, an embodiment of a compositematerial prior to pyrolysis, and an embodiment of an aligned CNT sheet.

FIG. 5A depicts the electrical conductivity of embodiments of startingmaterials and composite materials along a particular direction.

FIG. 5B depicts the electrical conductivity of embodiments of startingmaterials and composite materials along a particular direction.

FIG. 5C compares the electrical conductivity of one embodiment of acomposite material with other materials and graphite.

FIG. 6 depicts the temperature-dependent in-plane thermal conductivityof one embodiment of a composite material.

DETAILED DESCRIPTION

Composite materials, and methods of making composite materials, areprovided that include a ceramic material and carbon nanoscale fibers.The composite materials may [1] include a relatively high volumefraction of carbon nanoscale fibers, [2] be flexible, [3] have arelatively high electrical conductivity, [4] have a relatively hightensile strength, [5] have a relatively long fiber pullout, [6] have arelatively high thermal conductivity, and/or [7] a combination thereof.

It was surprisingly discovered that the composite materials herein mayinclude a ceramic material and a relatively high volume fraction ofcarbon nanoscale fibers, and/or embodiments of the composite materialsmay be flexible. In certain embodiments, the composite materials hereinmay be subjected to a rigorous flexibility test, such as the testsdescribed herein, without breaking. The flexibility tests mayunexpectedly have little or no effect on the flexural strength and/ormodulus of the composite materials. For example, the flexibility testsmay impact the flexural strength and/or modulus of the compositematerials by no more than 10%, 7.5%, 5%, 2.5%, or 1%. The “relativelyhigh” volume fraction of carbon nanoscale fibers in the compositematerials herein may be at least 35%, e.g., from 35% to 80%.

The composite materials provided herein, in embodiments, maysurprisingly have a relatively high electrical conductivity, at leastalong the direction of fiber alignment. For example, in one embodiment,the composite materials may have an electrical conductivity along thedirection of fiber alignment that is similar to that of graphite in thebasal plane.

Methods of Forming Composite Materials

The methods of forming a composite material provided herein may compriseproviding a carbon nanoscale fiber network which comprises a pluralityof substantially aligned carbon nanoscale fibers.

The phrase “carbon nanoscale fiber network”, as used herein, refers to amacroscopic aggregate of carbon nanoscale fibers. The carbon nanoscalefiber networks herein may be in the form of a sheet (i.e., film) orstrip (i.e., ribbon). The carbon nanoscale fiber networks generally mayhave any dimensions suited to a particular application. For example, thecarbon nanoscale fiber networks may have a length of about 10 cm toabout 10 m, a width of about 7 mm to about 10 mm, and a thickness ofabout 10 μm to about 50 μm. Other dimensions are envisioned, includinglengths and/or widths that exceed 10 m. Carbon nanoscale fiber networksare available commercially, or may be formed by techniques known in theart, such as dispersing carbon nanoscale fibers in a non-solvent andfiltering and/or evaporating the non-solvent. The carbon nanoscalefibers of the carbon nanoscale fiber networks may be substantiallyaligned prior to and/or after the formation of the carbon nanoscalefiber networks. For example, the carbon nanoscale fibers of a dispersionmay be aligned prior to filtering and/or evaporating a non-solvent,and/or the carbon nanoscale fiber networks may be stretched, asdescribed herein.

The carbon nanoscale fibers of the carbon nanoscale fiber network are“substantially aligned” when at least 20% of the carbon nanoscalefibers, by weight, are arranged within ±5° of parallel to a particularaxis. In some embodiments, at least 30%, at least 40%, at least 50%, atleast 60%, at least 70%, at least 80%, or at least 90%, by weight, ofthe plurality of carbon nanoscale fibers are arranged within ±5° ofparallel to a particular axis.

In embodiments, the providing of the carbon nanoscale fiber networkwhich comprises a plurality of substantially aligned carbon nanoscalefibers may include [1] providing a carbon nanoscale fiber network whichcomprises a plurality of randomly oriented carbon nanoscale fibers; and[2] stretching the carbon nanoscale fiber network to substantially alignthe plurality of randomly oriented carbon nanoscale fibers, wherein thestretching of the carbon nanoscale fiber network imparts the carbonnanoscale fiber network with a stretch ratio of about 10% to about 70%.In one embodiment, the stretch ratio is about 25% to about 45%. Inanother embodiment, the stretch ratio is about 35%.

The stretching may substantially align the plurality of randomlyoriented carbon nanoscale fibers. A plurality of carbon nanoscale fibersis “randomly oriented” when no more than 20% of the carbon nanoscalefibers, by weight, are arranged within ±5° of parallel to a particularaxis.

The stretching of the carbon nanoscale fiber network generally mayinclude applying a stretching force to the network. The stretching forcemay be applied along a single axis. For example, if the carbon nanoscalefiber network is in the form of a ribbon, a stretching force may beapplied to the ends of the ribbon, or, alternatively, along the entiresides of the ribbon. The stretching force generally may be sufficient toachieve any of the stretch ratios described herein.

The stretching force may be applied mechanically. The mechanicalstretching may be achieved by applying a stretching force to the carbonnanoscale fiber network with any means known in the art, including oneor more rollers, such as pinch rollers.

The stretching of the carbon nanoscale fiber networks may impart thecarbon nanoscale fiber networks with a stretch ratio of about 10% toabout 70%. The “stretch ratio” is determined by the following equation:

(D2−D1)/D1*100=Stretch Ratio

wherein D1 is the original dimension of the network along the axis towhich the stretching force is applied, and D2 is the new dimension ofthe network, after stretching, along the same axis. For example, if thenetwork is in the shape of a ribbon 10 m long and 0.5 m wide, and astretching force applied to the ends of the ribbon increases the lengthof the ribbon to 12 m, then the stretch ratio is 20%. In one embodiment,the stretch ratio is about 10% to about 70%. In another embodiment, thestretch ratio is about 10% to about 60%. In a further embodiment, thestretch ratio is about 10% to about 50%. In a still further embodiment,the stretch ratio is about 20% to about 50%. In some embodiments, thestretch ratio is about 25% to about 45%. In a particular embodiment, thestretch ratio is about 35%.

The stretching steps may increase the bulk density of the carbonnanoscale fiber networks. In embodiments, the stretching steps describedherein may increase the bulk density of the carbon nanoscale fibernetworks by about 20% to about 50%, about 30% to about 40%, or about35%. In other words, a carbon nanoscale fiber network having an originalbulk density of about 0.6 g/cm may have a bulk density of about 0.72g/cm (20% greater) to about 0.9 g/cm (50% greater) after stretching.

The methods provided herein may include infiltrating the carbonnanoscale fiber network with a liquid ceramic precursor. The carbonnanoscale fiber network that is infiltrated with the liquid ceramicprecursor may be a network that includes randomly oriented carbonnanoscale fibers, substantially aligned carbon nanoscale fibers, or acombination thereof.

The infiltration may be performed in a manner that permits the liquidceramic precursor to (i) adsorb to one or more surfaces of the carbonnanoscale fiber network, (ii) penetrate the carbon nanoscale fibernetwork, or (iii) a combination thereof. In one embodiment, the liquidceramic precursor penetrates the carbon nanoscale fiber network, and,therefore, is disposed in the internal structure of the carbon nanoscalefiber network. In another embodiment, the liquid ceramic precursor isadsorbed to one or more surfaces of the carbon nanoscale fiber network,and disposed in the internal structure of the carbon nanoscale fibernetwork. The liquid ceramic precursor may be disposed and/or dispersedevenly or unevenly on one or more surfaces or in the internal structure,respectively, of the carbon nano scale fiber network.

After the infiltrating of the carbon nanoscale fiber network with theliquid ceramic precursor, the liquid ceramic precursor may be adsorbedto one or both sides of the carbon nanoscale fiber network when, forexample, the carbon nanoscale fiber network is in the shape of a sheetor ribbon. The phrase “one or both sides” refers to the surfaces of thesheet or ribbon that are opposite each other and have the largestsurface areas of all the surfaces of the sheet or ribbon, respectively.In addition or alternative to “one or both sides”, the liquid ceramicprecursor may be adsorbed to one or more of the surfaces of the edges ofthe ribbon or sheet. The “surfaces of the edges” of the ribbon or sheetinclude those portions having one dimension defined by the thickness ofthe ribbon or sheet, respectively. The amount of liquid ceramicprecursor per surface area unit (e.g., cm², mm², etc.) of the one ormore surfaces of the carbon nanoscale fiber network may be substantiallyequal (±5%) or unequal.

The liquid ceramic precursor may penetrate the carbon nanoscale fibernetwork, and, therefore, be disposed in the internal structure of thecarbon nanoscale fiber network. The “internal structure” of a carbonnanoscale fiber network includes the network formed by the carbonnanoscale fibers and portions thereof that cannot be contacted bytouching an external surface of the carbon nanoscale fiber network. Forexample if three sheets of a carbon nanoscale fiber network werestacked, then the “internal structure” of the middle sheet is formed bythe carbon nanoscale fibers and portions thereof that do not contact thefirst and third sheets. The liquid ceramic precursor that penetrates thecarbon nanoscale fiber network may be adsorbed to carbon nanoscalefibers having at least a portion that exists beneath a surface of thecarbon nanoscale fiber network, and/or the disposed in the interstitialspace between the carbon nanoscale fibers having at least a portion thatexists beneath a surface of the carbon nanoscale fiber network. Theamount of liquid ceramic precursor per volume (e.g., mm³, μm³, etc.) inthe internal structure of the carbon nanoscale fiber networks may besubstantially equal (±5%) or unequal.

The infiltrating of the carbon nanoscale fiber network with the liquidceramic precursor may be performed using any techniques known in theart. For example, the infiltrating may be achieved with a filteringapparatus. The carbon nanoscale fiber network may be placed on thefiltering apparatus, and the liquid ceramic precursor may infiltrate thecarbon nanoscale fiber network as the liquid ceramic precursor contactsthe carbon nanoscale fiber network. The filtration may be assisted byvacuum pressure, positive pressure, or a combination thereof. The liquidceramic precursor also may be heated before and/or during theinfiltrating step. Heating the liquid ceramic precursor may lower itsviscosity, thereby easing the filtration and/or increasing the amount ofliquid ceramic precursor that infiltrates the carbon nanoscale fibernetwork.

Generally, a carbon nanoscale fiber network of any size may be subjectedto the infiltrating step. The size of the carbon nanoscale fiber networkmay be limited, in some instances, only by the size of the filteringapparatus. The filtering apparatus may be configured to operatecontinuously, thereby permitting the infiltration of large carbonnanoscale fiber networks, such as carbon nanoscale fiber networks havingat least one dimension of 10 m or more. Other sizes are envisioned.

The methods provided herein also may include curing the liquid ceramicprecursor to form a cured ceramic precursor. In embodiments, curing theliquid ceramic precursor comprises heating the carbon nanoscale fibernetwork that has been infiltrated with a liquid ceramic precursor to atemperature effective to cure the liquid ceramic precursor. In oneembodiment, curing the liquid ceramic precursor comprises heating thecarbon nanoscale fiber network infiltrated with a liquid ceramicprecursor to a temperature of about 100° C. to about 200° C., about 100°C. to about 180° C., about 120° C. to about 180° C., about 120° C. toabout 160° C., about 130° C. to about 150° C., or about 140° C. Thecarbon nanoscale fiber network infiltrated with a liquid ceramicprecursor may be heated for about 1 hour to about 48 hours, about 6hours to about 36 hours, about 12 hours to about 36 hours, or about 24hours. In particular embodiments, the liquid ceramic precursor comprisespolysilazane, and the carbon nanoscale fiber network infiltrated withthe liquid ceramic precursor is heated to about 140° C. for about 24hours to cure the liquid ceramic precursor.

The methods provided herein also may include pyrolyzing the curedceramic precursor. The pyrolysis may occur at any temperature for anytime effective to convert the cured liquid ceramic precursor to aceramic material. For example, the pyrolysis may occur at a temperatureof about 500° C. to about 1,500° C., about 800° C. to about 1,200° C.,or about 1,000° C. The pyrolysis also may occur under an inertatmosphere, such as a nitrogen atmosphere.

The infiltrating, curing, and pyrolyzing steps may be repeated one ormore times. In one embodiment, the infiltrating, curing, and pyrolyzingsteps are repeated once. In another embodiment, the infiltrating,curing, and pyrolyzing steps are repeated twice. Not wishing to be boundby any particular theory, it is believed that repeating theinfiltrating, curing, and pyrolyzing steps may increase the density ofthe resulting composite material. The density may be increased becauseat least a portion of any pores and/or empty spaces in a compositematerial at the conclusion of a pyrolysis step may be at least partiallyfilled by liquid ceramic precursor when the infiltrating, curing, andpyrolyzing steps are repeated.

In embodiments, the methods provided herein comprise [1] providing acarbon nanoscale fiber network which comprises a plurality ofsubstantially aligned carbon nanoscale fibers; [2] infiltrating thecarbon nanoscale fiber network with a first amount of a liquid ceramicprecursor; [3] curing the first amount of the liquid ceramic precursorto form a cured ceramic precursor; [4] pyrolyzing the cured ceramicprecursor to form the composite material; [5] infiltrating a secondamount of the liquid ceramic precursor into the composite material; [6]curing the second amount of the liquid ceramic precursor to form asecond amount of a cured ceramic precursor; and [7] pyrolyzing thesecond amount of the cured ceramic precursor, wherein the volumefraction of the carbon nanoscale fibers in the composite material is atleast 35%.

In one embodiment, the methods provided herein comprise [1] providing acarbon nanoscale fiber network which comprises a plurality ofsubstantially aligned carbon nanoscale fibers; [2] infiltrating thecarbon nanoscale fiber network with a first amount of a liquid ceramicprecursor comprising polysilazane; [3] curing the first amount of theliquid ceramic precursor comprising polysilazane to form a cured ceramicprecursor; [4] pyrolyzing the cured ceramic precursor to form a carbonnanoscale fiber/silicon carbonitride composite material.

Composite Materials

Composite materials also are provided herein, which may comprise aceramic material dispersed in a carbon nanoscale fiber network whichcomprises a plurality of substantially aligned carbon nanoscale fibers.In one embodiment, the ceramic material comprises silicon carbonitride.The ceramic material may be unevenly or at least substantially evenlydispersed in and/or on the carbon nanoscale fiber network.

The volume fraction of carbon nanoscale fibers in the compositematerials may be at least 35%. In one embodiment, the volume fraction ofcarbon nanoscale fibers in the composite materials is about 40% to about80%. In another embodiment, the volume fraction of carbon nanoscalefibers in the composite materials is about 40% to about 75%. In afurther embodiment, the volume fraction of carbon nanoscale fibers inthe composite materials is about 50% to about 70%. In a particularembodiment, the volume fraction of carbon nanoscale fibers in thecomposite materials is about 60%. The volume fraction of carbonnanoscale fibers in the composite materials may be determined bycalculating the relative density change as the materials are produced,as explained, for example, at Example 3.

The composite materials provided herein may have an electricalconductivity of at least 1.75×10⁴ S/m, according to the measurementtechnique described at Example 4. Therefore, a composite material has anelectrical conductivity of at least 1.75×10⁴ S/cm when its conductivityalong [1] its length and/or [2] the direction perpendicular to thelength is at least 1.75×10⁴ S/cm. In one embodiment, the compositematerials have an electrical conductivity of about 1.8×10⁴ S/m to about3.0×10⁴ S/m. In another embodiment, the composite materials have anelectrical conductivity of about 2.0×10⁴ S/m to about 3.0×10⁴ S/m. In afurther embodiment, the composite materials have an electricalconductivity of about 2.0×10⁴ S/m to about 2.5×10⁴ S/m. In an additionalembodiment, the composite materials have an electrical conductivity ofabout 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

In embodiments, the volume fraction of carbon nanoscale fibers in thecomposite material is at least 35%, about 40% to about 80%, about 40% toabout 75%, about 50% to about 70%, or about 60%; and the electricalconductivity of the composite material is at least 1.75×10⁴ S/cm.

In embodiments, the volume fraction of carbon nanoscale fibers in thecomposite material is at least 35%, about 40% to about 80%, about 40% toabout 75%, about 50% to about 70%, or about 60%; and the electricalconductivity of the composite material is about 1.8×10⁴ S/m to about3.0×10⁴ S/m.

In embodiments, the volume fraction of carbon nanoscale fibers in thecomposite material is at least 35%, about 40% to about 80%, about 40% toabout 75%, about 50% to about 70%, or about 60%; and the electricalconductivity of the composite material is about 2.0×10⁴ S/m to about3.0×10⁴ S/m.

In embodiments, the volume fraction of carbon nanoscale fibers in thecomposite material is at least 35%, about 40% to about 80%, about 40% toabout 75%, about 50% to about 70%, or about 60%; and the electricalconductivity of the composite material is about 2.0×10⁴ S/m to about2.5×10⁴ S/m.

In embodiments, the volume fraction of carbon nanoscale fibers in thecomposite material is at least 35%, about 40% to about 80%, about 40% toabout 75%, about 50% to about 70%, or about 60%; and the electricalconductivity of the composite material is about 2.0×10⁴ S/m to about2.25×10⁴ S/m.

The composite materials provided herein also may be flexible. Acomposite material is “flexible” when, according to the flexibility testdescribed at Example 2, it can be bent at least 500 times at a bendingpercentage of at least 50% without breaking. The composite material mayreturn to a shape that is at least substantially similar to its originalshape after each bending. In one embodiment, the composite materialsprovided herein can be elastically deformed (i.e., bent) at least 500times at a bending percentage of at least 60%, at least 70%, at least75%, at least 80%, at least 90%, or 100% without breaking. In anotherembodiment, the composite materials provided herein can be bent at least1,000 times at a bending percentage of at least 60%, at least 70%, atleast 75%, at least 80%, at least 90%, or 100% without breaking.

In embodiments, the composite material is flexible; the volume fractionof carbon nanoscale fibers in the composite material is at least 35%,about 40% to about 80%, about 40% to about 75%, about 50% to about 70%,or about 60%; and the electrical conductivity of the composite materialis at least 1.75×10⁴ S/cm.

In embodiments, the composite material is flexible; the volume fractionof carbon nanoscale fibers in the composite material is at least 35%,about 40% to about 80%, about 40% to about 75%, about 50% to about 70%,or about 60%; and the electrical conductivity of the composite materialis about 1.8×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the volume fractionof carbon nanoscale fibers in the composite material is at least 35%,about 40% to about 80%, about 40% to about 75%, about 50% to about 70%,or about 60%; and the electrical conductivity of the composite materialis about 2.0×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the volume fractionof carbon nanoscale fibers in the composite material is at least 35%,about 40% to about 80%, about 40% to about 75%, about 50% to about 70%,or about 60%; and the electrical conductivity of the composite materialis about 2.0×10⁴ S/m to about 2.5×10⁴ S/m.

In embodiments, the composite material is flexible; the volume fractionof carbon nanoscale fibers in the composite material is at least 35%,about 40% to about 80%, about 40% to about 75%, about 50% to about 70%,or about 60%; and the electrical conductivity of the composite materialis about 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

The composite materials provided herein may have a tensile strength ofat least 350 MPa, at least 400 MPa, or at least 450 MPa, as determinedby the method described at Example 3. In embodiments, the compositematerial has a tensile strength of about 400 MPa to about 800 MPa, about400 MPa to about 700 MPa, about 450 MPa to about 650 MPa, or about 500MPa to about 600 MPa. In one embodiment, the composite material isflexible, and has a tensile strength of at least 350 MPa, at least 400MPa, or at least 450 MPa. In further embodiments, the composite materialis flexible, and has a tensile strength of about 400 MPa to about 800MPa, about 400 MPa to about 700 MPa, about 450 MPa to about 650 MPa, orabout 500 MPa to about 600 MPa.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 350 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isat least 1.75×10⁴ S/cm.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 350 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 1.8×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 350 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 350 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.5×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 350 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 400 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isat least 1.75×10⁴ S/cm.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 400 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 1.8×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 400 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 400 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.5×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 400 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 450 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isat least 1.75×10⁴ S/cm.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 450 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 1.8×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 450 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 450 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.5×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 450 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 500 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isat least 1.75×10⁴ S/cm.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 500 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 1.8×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 500 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 3.0×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 500 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.5×10⁴ S/m.

In embodiments, the composite material is flexible; the tensile strengthof the composite material is at least 500 MPa; the volume fraction ofcarbon nanoscale fibers in the composite material is at least 35%, about40% to about 80%, about 40% to about 75%, about 50% to about 70%, orabout 60%; and the electrical conductivity of the composite material isabout 2.0×10⁴ S/m to about 2.25×10⁴ S/m.

Carbon Nanoscale Fibers

As used herein, the phrase “carbon nanoscale fiber” refers to a thin,greatly elongated solid material comprising carbon, typically having across-section or diameter of less than 500 nm. The carbon nanoscalefibers may have an aspect ratio of about 100:1 to about 100,000:1. Theaspect ratio may be greater than 100,000:1. In one embodiment, thecarbon nanoscale fibers have a length of about 0.5 mm to about 1.2 mm,and a diameter of about 3 nm to about 10 nm. In a particular embodiment,the carbon nanoscale fibers have a length of about 1.0 mm, and adiameter of about 3 nm to about 8 nm. In another embodiment, the carbonnanoscale fibers have a length of about 1.0 mm, and a diameter of about6 nm to about 8 nm.

The carbon nanoscale fibers may include CNTs. In embodiments, the carbonnanoscale fibers include single-wall carbon nanotubes (SWNTs),multi-wall carbon nanotubes (MWNTs), carbon nanofibers, and combinationsthereof. In some embodiments, the CNTs are SWNTs. In particularembodiments, the CNTs are MWNTs. In still further embodiments, the CNTsare a mixture of SWNTs and MWNTs. In additional embodiments, the CNTsare a mixture of SWNTs, MWNTs, and carbon nanofibers.

As used herein, the phrase “carbon nanotubes” and the abbreviation“CNTs” generally refer to tubular graphite, which may be capped withfullerene structures. The CNTs may be a synthetic material having a widemolecular weight range that depends substantially on the diameter andlength of the CNTs. CNTs are commercially available from companies suchas General Nano, LLC (Cincinnati, Ohio, USA) and Nanocomp TechnologiesInc. (NH, USA), or can be made using techniques known in the art. TheCNTs can be pristine, in which the carbon fullerene tubes have fullereneend caps, or the CNTs can be non-pristine, for example, where thepristine CNTs have been chemically or mechanically altered (e.g.,chopped) and then optionally functionalized to convert dangling carbonatoms to different functional groups, such as carbonyl or other oxygencontaining groups. The sidewalls of the CNTs also may be functionalizedto include one or more functional groups. The CNTs, in embodiments, alsoinclude one or more other nanomaterials, such as graphene, metalnanoparticles, or a combination thereof. In embodiments, the CNTs arepristine MWNTs. In other embodiments, the CNTs are non-pristine MWNTs.In some embodiments, the CNTs include a mixture of pristine MWNTs andpristine SWNTs. In still further embodiments, the CNTs include a mixtureof pristine MWNTs and non-pristine SWNTs, or vice versa. In oneembodiment, the CNTs are pristine SWNTs. In another embodiment, the CNTsare non-pristine SWNTs. In each of the foregoing embodiments, thesidewalls of at least a portion of the SWNTs, MWNTs, or a combinationthereof may be functionalized. As used herein, the phrase “carbonnanofibers” refers to filamentous fibers that resemble whiskers ofmultiple graphite sheets or MWNTs.

In one embodiment, the carbon nanoscale fiber network comprises CNTs,and the carbon nanoscale fiber network is a buckypaper. Buckypapers maybe made through the dispersion of CNTs in suspension followed by afiltration or evaporation process, or stretching or pushing synthesizednanotube “forests” to form sheets or strips.

The carbon nanoscale fiber networks herein may be rolled and/or pressedaccording to techniques known in the art. A rolling and/or pressingtechnique may be performed at any time during, before, and/or after anyof the steps of the methods described herein are performed. For example,a rolling and/or pressing technique may be performed before, after, orboth before and after the stretching of the carbon nanoscale fibernetworks. The pressing may be a hot-pressing technique. In oneembodiment, the carbon nanoscale fibers comprise CNTs, and the carbonnanoscale fiber network is stretched as described herein, and thensubjected to hot-pressing.

Not wishing to be bound by any particular theory, it is believed thatthe stretching steps described herein or the stretching steps describedherein in combination with at least one of rolling and pressing mayachieve relatively dense packing and improved alignment of the carbonnanoscale fibers of the carbon nanoscale fiber networks.

Functionalization of the carbon nanoscale fibers and/or the carbonnanoscale fiber networks may be performed at any time during, before,and/or after any of the steps of the methods described herein areperformed. Surface functionalization may be achieved by subjecting thecarbon nanoscale fibers and/or carbon nanoscale fiber networks tomicrowaves, plasma, electron beam, chemical functionalization, or acombination thereof. Not wishing to be bound by any particular theory,it is believed that the surface functionalization techniques may improveat least one of the mechanical and electrical properties of the carbonnanoscale fibers and/or carbon nanoscale fiber networks.

Liquid Ceramic Precursors

The liquid ceramic precursors generally may be or include any substancecapable of being converted to a ceramic material. In embodiments, theliquid ceramic precursor comprises one or more polysilazanes,polysiloxanes, polyborosiloxanes, polyborosilanes, polyborosilazanes,polycarbosiloxanes, polycarbosilanes, or combinations thereof. In oneembodiment, the liquid ceramic precursor comprises a polysilazane. Thepolysilazane may be converted to silicon carbonitride upon curing and/orpyrolysis.

Applications

Due at least in part to their relatively high electrical conductivity,the composite materials provided herein may be used in manymultifunctional applications, such as electromagnetic interference (EMI)shielding.

Due at least in part to their lightweight structures, high mechanicalstrength, high-temperature resistance, chemical inertness, or acombination thereof, the composite materials provided herein may playimportant roles in many defense-related applications, such asliquid-fueled rocket engine combustion devices (thrust chambers andnozzles), aircraft brakes, and thermal protection systems on hypersonicvehicles. The composite materials provided herein may be useful as analternative for high-temperature, heavy metallic components and/orbrittle ceramics in defense-related systems.

The composite materials provided herein may provide a new template forthe design of high-temperature flexible electronics. Examples include,but are not limited to, high-temperature electronic components that arecapable of adapting to the shape change of aerospace components whilemaintaining key functionality, such as sensing and actuating. Theproperties of the composite materials, especially the conductivities,may be adjusted, in embodiments, both along the in-plane direction andthe through-thickness direction, by varying the material compositionduring the fabrication process.

When the composite materials are in the form of a flexible membrane, thecomposite materials can be used in thermal management, energy, and/orsensing in a number of environments, including harsh and/or extremeenvironments.

In the descriptions provided herein, the terms “includes,” “is,”“containing,” “having,” and “comprises” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to.” When methods and composite materials are claimed ordescribed in terms of “comprising” various components or steps, thecomposite materials and methods can also “consist essentially of or”consist of the various components or steps, unless stated otherwise.

The terms “a,” “an,” and “the” are intended to include pluralalternatives, e.g., at least one. For instance, the disclosure of “aliquid ceramic precursor,” “a carbon nanoscale fiber,” “a carbonnanoscale fiber network”, and the like, is meant to encompass one, ormixtures or combinations of more than one liquid ceramic precursor,carbon nanoscale fiber, carbon nanoscale fiber network, and the like,unless otherwise specified.

Various numerical ranges may be disclosed herein. When Applicantdiscloses or claims a range of any type, Applicant's intent is todisclose or claim individually each possible number that such a rangecould reasonably encompass, including end points of the range as well asany sub-ranges and combinations of sub-ranges encompassed therein,unless otherwise specified. Moreover, all numerical end points of rangesdisclosed herein are approximate. As a representative example, Applicantdiscloses, in one embodiment, that the volume fraction of carbonnanoscale fibers in the composite material is about 50% to about 70%.This range should be interpreted as encompassing weight percentages in arange of about 50% to about 70%, and further encompasses “about” each of51%, 52%, 53%, 54%, 55%, 56% 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%,65%, 66%, 67%, 68%, and 69%, including any ranges and sub-ranges betweenany of these values.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other aspects, embodiments, modifications,and equivalents thereof which, after reading the description herein, maysuggest themselves to one of ordinary skill in the art without departingfrom the spirit of the present invention or the scope of the appendedclaims. Thus, other aspects of this invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein.

Example 1—Formation of a Composite Material

One embodiment of a composite material was made according to thefollowing procedure. Carbon nanotube sheets of randomly orientedmulti-wall CNTs (Nanocomp Technologies, Inc.) were mechanicallystretched. The multi-wall CNTs had an average diameter of about 6 nm toabout 8 nm, and an average length of about 1 mm. The sheet wasmechanically stretched until a stretch ratio of 35% was achieved. Notwishing to be bound by any particular theory, it was believed that theextremely high aspect ratio (>100,000) of the carbon nanotubes of thisexample contributed to the high ductility and super elasticity of thecarbon nanotube networks, which allowed the network to be highly alignedby mechanical stretching process.

The mechanical stretching resulted in an aligned carbon nanotube sheethaving a width of about 8 mm and a thickness of about 0.041 mm. Thealigned carbon nanotubes sheet was cut to form a sheet about 38 mm inlength.

In this example, polysilazane (PSZ) (KION Defense Technologies, Inc.)was used as the liquid preceramic precursor of silicon carbonitride(SiCN), and 4 wt. % dicumyl peroxide (Sigma-Aldrich Co.) was used as thethermal initiator. PSZ is a low viscosity liquid thermosetting resinwith repeat units in which silicon and nitrogen atoms are bonded in analternating sequence.

Not wishing to be bound by any particular theory, it was believed thatthe CNTs of the aligned carbon nanotube sheet formed a relatively tightmesh that included small pockets of empty space, which were infiltratedby the liquid polysilazane precursor of this example.

In this example, the liquid precursor was forced into these empty spaceswith the aid of a vacuum. The sample was then cured into a solidpreceramic state by thermal crosslinking at 140° C. for 24 hours. Thisprocess was based on the wet infiltration of liquid ceramic polymericprecursors, which is commonly called polymer impregnation and pyrolysis(PIP) process.

Following curing, it was believed that a solid mixture CNTs/PSZ wasobtained where at least a portion of the carbon nanotubes were at leastpartially wrapped by the precursor.

A pyrolysis process was then performed at elevated temperatures (1000°C.) in nitrogen atmosphere. It was believed that the pyrolysis processdecomposed the cured solid precursor in the carbon nanotube sheets intoamorphous silicon carbonitride ceramics, thereby yielding a flexibleceramic nanocomposite. Not wishing to be bound by any particular theory,it was believed that at least a portion of the CNTs may have beenexposed as a result of the pyrolysis process.

Smaller sized pores and spaces still existed internally, and these werefurther filled with a reiteration of the polymer impregnation andpyrolysis (PIP) process to obtain near fully-densified nanocomposites.

Therefore, the foregoing infiltration, curing, and pyrolysis steps wererepeated once. The repeating of the steps, although optional, wasbelieved to fill in and/or decrease the size of the spaces and/or poresthat existed in the composite material prior to the repeating of theinfiltration, curing, and pyrolysis steps.

As a result, four stages existed in the process of this example: (a)aligned CNTs sheet, (b) carbon nanotube sheet infiltrated bypolysilazane (CNTs/PSZ), (c) carbon nanotube/silicon carbonitridenanocomposites (CNTs/SiCN1), and (d) carbon nanotube/siliconcarbonitride nanocomposites with one more PIP process (CNTs/SiCN2).

Not wishing to be bound by any particular theory, it was believed thatthe high aspect ratio (>100,000), high ductility, and/or superelasticity of the carbon nanotubes allowed them to be aligned by themechanical stretching method. The resulting aligned carbon nanotubesheet was believed to contain a closer packing of carbon nanotubes withfewer voids and lower porosity that the carbon nanotube sheet ofrandomly oriented carbon nanotubes.

The original dimensions (length×width×thickness) of the aligned CNTssheet were 38 mm×8 mm×0.041 mm. After the foregoing infiltration andcuring steps, it was believed that the pores and gaps were substantiallyfilled with the solidified polysilazane, which was believed to at leastpartially wrap the carbon nanotubes.

During the polymer-ceramic conversion from polysilazane to siliconcarbonitride, the density increased significantly from 1.1 g·cm⁻³ to2.23 g·cm⁻³, and around 28% linear shrinkage occurred, which resulted ina large volume reduction.

FIG. 1 depicts the sample size change and the shrinkage percentage fromCNTs/PSZ to CNTs/SiCN1 along the length, width, and thicknessdirections. As carbon nanotubes exhibit highly anisotropic mechanicaland functional properties, it was believed that the aligned carbonnanotube sheet maintained the same anisotropic property, which may havecaused the different shrinkage rates along these three directions. Theshrinkage rate along the length direction was believed to be small asthe cylindrical layer-structure of carbon nanotubes likely confined thecarbon nanotube sheet's structure and restricted the shrinkage.

The size change that occurred during the pyrolysis process of thisexample also is summarized in the following table:

Cross-sectional Length/mm Width/mm Thickness/mm Area/mm² CNTs/PSZ 38.587.93 0.041 0.33 CNTs/SiCN1 38.55 6.87 0.028 0.19 Shrinkage 0.08% 13.37%31.71% 42.42% Percentage

After the infiltration and curing process, the original dimensions(length×width×thickness) of CNTs/PSZ were 38.58 mm×7.93 mm×0.041 mm. Thedimensions of CNTs/SiCN1 became 38.55 mm×6.87 mm×0.028 mm due to thepyrolysis process, indicating that the cross-sectional area along thealignment direction decreased 42.42%. It was believed that the shrinkagedid not affect the alignment degree of carbon nanotubes; instead, itappeared to result in closer contact between each carbon nanotube.

Not wishing to be bound by any particular theory, it was believed thatthe sample size shrinkage came from the pyrolysis of the precursorinside each gap between pairs of carbon nanotubes. This was believed tocause or at least contribute to the noticeable shrinkage along the widthand thickness directions as carbon nanotubes stacked onto each otheralong these two directions, which made the cross section (vertical tothe carbon nanotube alignment direction) decrease and the carbonnanotubes stack more closely together. SEM images of the peeledCNTs/SiCN1 revealed that the material exhibited near perfect carbonnanotube alignment, which was believed to help efficient load-transfer.The shrinkage was believed to lead to closer contact between eachadjacent pair of carbon nanotubes. The SEM images of the carbonnanotubes assembly in CNT sheets and CNTs/SiCN1, respectively, showedthat some bunches of CNTs having a diameter around 150 nm were assembledtogether during the pyrolysis process. Some pores could also be found inCNTs/SiCN1, therefore, the composites could be densified by repeatingthe foregoing polymer impregnation and pyrolysis (PIP) processing steps.

The materials also were characterized with Raman spectroscopy, which wascarried out on a Renishaw inVia micro-Raman system using a 785 nmexcitation wavelength (1.58 eV) diode laser. Typical laser power was 0.5mW with a 50× magnification objective lens, and the laser beam size wasaround 1 mm in diameter. Peaks at 1340 cm⁻¹ (D band) and 1589 cm⁻¹ (Gband) were the main features of sp2 crystalline graphitic structures. Apeak at 2660 cm⁻¹ (G′ band) was believed to reveal the limited layernature of the carbon nanotubes in their as-received state and in thecomposite materials. A relatively lower G′ band in the composites wasbelieved to suggest the possible overlapping of carbon nanotubes.Compared with aligned carbon nanotubes, the intensity of the D bandbecame much higher in CNTs/SiCN1 and CNTs/SiCN2 composites. It wasbelieved that the defective nature of CNTs in the composites may beascribed to the changes in Raman analysis. It also was believed that thereaction between carbon nanotubes and polysilazane during the pyrolysisprocess at high temperatures may have introduced certain defects.

The flexibility, mechanical properties, and microstructure of thecomposite materials of this example were characterized and studied, asexplained in the following examples, along with the toughening mechanismof carbon nanotubes in the composite materials.

Example 2—Flexibility Test

A flexibility test of the composite material of Example 1 was carriedout on a motorized linear stage (Zaber Technologies Inc.). Samples ofthe composite material of Example 1 were held between two cylindricalrods. One of the rods was movable, and the other was stationary.

Surprisingly, the samples of Example 1 were bent 1000 times to 50% and75% of their original length without breakage. Moreover, it wasunexpectedly observed that the flexural strength and modulus of thesamples were not substantially diminished as a result of the flexibilitytest.

During the flexibility test of this example, the bending percentage wasmonitored by the travel distance of the movable rod and the machine wasstopped at a specific bending percentage. In this example, 50% and 75%were the two bending percentages tested. The “bending percentage” wasthe percentage of the sample's length traveled by the moving rod, i.e.,the movable rod would move 7.5 mm towards the stationary rod if acomposite material sample 10 mm long were placed between the rods. Thebending tests of this example were repeated 500 and 1000 times. Thetravel distance and repetition times of the movable rod were controlledby a step motor.

During the flexibility test, two factors were controlled: the bendingpercentage (a) and the number of bending times. The bending percentageis described by the following equation:

$\alpha = {\frac{l_{0} - l_{1}}{l_{0}} \times 100\%}$

where l₀ is the original length of the composite material, and l₁ is thesmallest distance between the two rods during the flexibility test,which was monitored by the travel distance of the movable rod.

The detailed test conditions and resultant mechanical properties of thecomposite materials are provided in the following table:

Tensile Bending Percentage Strength Young's Modulus Item (α) Times (MPa)(GPa) 1 0 0 501.34 ± 7.53 101.24 ± 1.22  2 50% 500 511.51 ± 4.06 97.11 ±1.56 3 50% 1000 504.01 ± 5.23 92.54 ± 1.85 4 75% 1000 495.51 ± 6.9197.32 ± 5.43

As an example, for Item 4 of the foregoing table (75%, 1000 times) thelength of the composite material was 38 mm, and the composite materialwas pressed until the distance between the two rods was 9.5 mm. Themovable rod was moved back and forth 1000 times under this condition.

Due to the flexibility of the composite materials, the samples also werebent with fingertip pressure sufficient to cause the two ends of thesample to touch each other. After the bending deformation, the flexibleceramic nanocomposites were able to reflect back fully to the originalshape.

Not wishing to be bound by any particular theory, it was believed thatthe flexible ceramic nanocomposites showed two features in this example:thin structure and excellent toughening effect of carbon nanotubes. Thethin structure was believed to make the flexural rigidity very small,which is defined as the force couple required to bend a structure, asshown in the following equation:

$D = \frac{{Eh}^{3}}{12\left( {1 - v^{2}} \right)}$

wherein D is flexural rigidity, E is the Young's modulus, v is thePoisson's ratio, and h is the thickness. The flexural rigidity wasdetermined by the Young's modulus, Poisson's ratio, and the thickness.Taken from the experimental results of the composite materials ofExample 1, the Young's modulus was about 110 GPa, and the thickness wasabout 28 μm. The Poisson's ratio was taken as 0.35, so the flexuralrigidity of the composite materials was about 2.3×10⁻⁴ Pa·m³, whichindicated that the composite materials were easily bendable.

FIG. 2A depicts the mechanical properties of the composite material ofExample 1 before (Item 1 of the foregoing Table) and during the variousflexibility tests (Items 2-4 of the foregoing Table). The sample notsubjected to a flexibility test (Item 1 of the foregoing Table) islabeled “None” at FIG. 2A. All the samples initially exhibited a linearstress-strain relationship. The Young's Modulus of each sample, which isdepicted at FIG. 2B, was obtained from the slope of the linear portionof the stress-strain curve. Not wishing to be bound by any particulartheory, it was believed that due to the toughening of CNTs in thecomposite materials, deviation from the linear response was observed inthe stress-strain curves as the load continued.

The tensile strength and modulus of the composite materials after theflexibility test were substantially maintained at a constant levelcompared to the sample not subjected to the flexibility testing. SEMimages of the composite materials after the flexibility test (75%, 1000times) showed that no apparent crack appeared on the surface of thecomposite materials after the flexibility test. Similar fracture surfaceand carbon nanotube pullout phenomenon also were observed in the SEMimages of the composite materials subjected to the flexibility test. Theflexibility tests of this example, therefore, were believed todemonstrate that the composite materials of Example 1 at leastsubstantially retained the original mechanical properties andmicrostructures, which was believed to indicate excellent complianceand/or durability.

Example 3—Mechanical Properties and Characterization of Microstructures

The mechanical properties of the composite materials of Example 1 weretested according to the following procedures.

A tensile strength test was carried out to measure the mechanicalproperties of the aforementioned materials during the fabricationprocess. The tensile strength measurements were conducted by a tensilemachine (AGS-J, Shimadzu Scientific, Inc.) with a crosshead speed of 0.5mm·min⁻¹. The strain change was recorded by a non-contact videoextensometer DVE-201. To meet the tensile test requirement, theeffective size of the sample was controlled to 2 mm in width and 15 mmin gauge length approximately.

Five samples were used in the mechanical test to ensure repeatability.The tensile strength was obtained by taking the average value of thefive tests, and Young's modulus was obtained from the slope of eachlinear plot, as explained below. P 1000 grade sand paper tabs were usedto make dog-bone shaped samples, and meet the tensile test standard.

The microstructures of the flexible ceramic nanocomposites after thetensile strength test were observed using a scanning electron microscope(SEM, JEOL JSM-7401F).

FIG. 3A depicts the tensile stress-strain curves of the four materialstested in this example: [1] the aligned CNT sheet of Example 1 (“CNTssheet”), [2] the aligned CNT sheet infiltrated with cured PSZ(“CNTs/PSZ”), [3] the aligned CNT sheet with cured PSZ after pyrolysis(“CNTs/SiCN1”), and [4] the composite material of Example 1(“CNTs/SiCN2”).

All four curves initially exhibited a linear stress-strain relationship.The linear portion of each curve was the elastic region, and Young'smodulus was obtained from the slope of the linear fits depicted by thedotted line at FIG. 3A.

A comparison of the Young's modulus and mass increase is depicted atFIG. 3B. From the mass increase and size change of the four materials(i.e., from CNTs sheet to CNTs/SiCN2), the volume fraction of carbonnanotubes in the composite material of Example 1 was calculated to beabout 60%.

Not wishing to be bound by any particular theory, it was believed thatthe aligned carbon nanotube sheet of FIG. 3A exhibited a nonlinearelastic response due, at least in part, to its high ductility. It wasbelieved that due to the fact that the carbon nanotube sheet of Example1 had been aligned by mechanical stretching, the sheet had a hightensile strength along the alignment direction. The tensile strength ofthe CNTs sheet was 240.78±9.64 MPa. The relatively large failure strainin the aligned CNTs sheet was believed to result from the carbonnanotubes' intrinsic flexibility and high failure elongation, which alsowas believed to result in the Young's modulus of 9.14±0.47 GPa along thealigned direction, as shown at FIG. 3A and FIG. 3B.

The CNTs/PSZ was a type of thermoset polymer matrix composite. Notwishing to be bound by any particular theory, it was believed that adeviation from the linear response in the stress-strain curve as theload continued was observed at FIG. 3A for CNTs/PSZ because of thetoughening of carbon nanotubes for CNTs/PSZ.

During the tensile test, it was believed that the load was transferredfrom the polymer matrix to the carbon nanotubes via interfacial bonding.The tensile strength and Young's modulus of CNTs/PSZ reached 318.12±8.47MPa and 23.16±2.46 GPa, respectively, which was 132% and 253% of thealigned CNTs sheet's values, respectively.

After the pyrolysis process, the tensile strength of CNTs/SiCN1 reached501.34±7.55 MPa, which was 208% of the aligned CNTs sheet's value, asshown at FIG. 3A. The cured polysilazane in the materials was believedto transform into silicon carbonitride ceramic during the pyrolysisprocess. Due to the fact that ceramics typically have high elasticmodulus, the pyrolysis was believed to result in the significantenhancement of the Young's modulus of CNTs/SiCN1, which was 101.24±1.22GPa, compared to the aligned CNTs sheet.

Further polymer impregnation and pyrolysis was believed to improve themechanical strength of the nanocomposites of Example 1 through enhanceddensification (tensile strength: 536.33±7.23 MPa and Young's modulus:117.78±3.21 GPa), which was a 223% and 1289% improvement over thepristine aligned carbon nanotubes sheet, respectively.

The nonlinear behavior of CNTs/SiCN1 and CNTs/SiCN2 was also observed intheir tensile stress-strain curves. The inset plot of FIG. 3A wasbelieved to at least partially explain the deviation from the linearresponse of the tensile stress-strain curves and the tougheningmechanism of carbon nanotubes in CNTs/SiCN1 and CNTs/SiCN2.

SEM images of the fracture surfaces of the four samples were collectedafter the tensile test. The SEM image of the aligned CNT sheet depicteda sheet break with a large width of the sheet tapered to several smallbundles of CNTs, which was believed to indicate a non-brittle fracture.The carbon nanotubes pullout was observed at the fracture surface of anSEM image of the CNTs/PSZ material. Not wishing to be bound by anyparticular theory, it was believed that the load transfer between carbonnanotubes and the polymer matrix played a critical role for the failuremodes. A unique fracture surface was observed for the carbon nanotubesreinforced ceramic matrix composites. The fracture surfaces showed longpullouts of carbon nanotubes. The SEM images also demonstrated that thethickness of CNTs/PSZ was much larger than the thicknesses of CNTs/SiCN1and CNTs/SiCN2, which was believed to verify the size change thatoccurred during the pyrolysis process.

SEM images also were used to determine the length of carbon nanotubespullout of the samples after the tensile test. Surprisingly, the lengthof the carbon nanotubes pullout was about 15 μm for CNTs/PSZ,CNTs/SiCN1, and CNTs/SiCN2.

Conventional toughening mechanisms in fiber reinforced ceramiccomposite, such as fiber pullout, debonding, and sliding at thefiber/matrix interface, were believed to explain these phenomena. It wasbelieved that in fiber reinforced brittle ceramic matrix compositesystems, the energy consumed by the fiber pullout, debonding, andsliding against the interface stress between fiber and ceramic matrixmostly contributed to the toughening of the samples. It has beendemonstrated that the energy dissipated during the fiber-ceramicdebonding and fiber pullout can be calculated by Equations (1) and (2),respectively:

$\begin{matrix}{G_{debond} = \frac{2V_{f}{LG}_{i}}{r}} & (1) \\{G_{pullout} = \frac{V_{f}L^{2}\tau_{i}}{r}} & (2)\end{matrix}$

where V_(f) is the fiber volume fraction, L is the pullout length, σ_(i)is the interface fracture energy, τ_(i) is the interfacial shear stress,and r is the fiber radius.

Equations (1) and (2) also were believed to provide a generalunderstanding about the toughening factors of carbon nanotubes on aceramic matrix, such as the volume fraction of carbon nanotubes, theinterface strength between carbon nanotubes and silicon carbonitridematrix, and the length of carbon nanotubes pullout. As explained atExample 3, the volume fraction of carbon nanotubes in the ceramic matrixcomposites of Example 1 was 60%. The length of carbon nanotubes pullout,as determined by the SEM images, was about 15 μm.

Therefore, the following experimental values were used in an analysisrelying on Equations (1) and (2): V_(f)=60%, L=15 μm, and r=8 nm. Theinterfacial shear stress (τ_(i)) was taken from a literature value (10MPa), and an estimate of σ_(i)=4 J/m² for glass matrix materials wasused in the calculations. Compared to carbon fiber reinforced siliconcarbide composites (C_(f)/SiC) and other carbon nanotubes/ceramiccomposites, the σ_(debond) and σ_(pullout) of the composite material ofExample 1 was relatively large, as shown at the following table:

V_(f) Sample (%) L (μm) R (nm) G_(debond) (J/m²) G_(pullout) (J/m²)C_(f)/SiC³⁰ 40 100 7 × 10³ 45.1 5.71 × 10³ CNTs/SiO₂ ³⁹ 10 0.1 10 8 1Example 1 60 15 8 9 × 10³ 1.68 × 10⁵

Not wishing to be bound by any particular theory, it was believed that asignificantly longer pullout length and a high volume fraction of carbonnanotubes can cause substantial increases in σ_(pullout) and σ_(debond),thereby increasing the toughening effect.

Example 4—Electrical Properties and Anisotropy

The electrical properties of the composite material of Example 1 weretested. The electrical properties were obtained by a four probe method,and the results demonstrated anisotropy along the CNT aligned directionand the vertical direction. The results are depicted at FIG. 4.

Surprisingly, the electrical resistivity of the ceramic membrane wasonly a quarter of the value of the CNT sheet.

Specifically, the electrical conductivity was measured using thefour-probe method (Jandel universal probe system with Keithley 2002 MEMmultimeters). It was tested five times for each sample and theelectrical conductivity was obtained by calculating the average value.The influence of the pyrolysis process on the anisotropy of theelectrical properties was investigated.

In this example, the high volume fraction, high aspect ratio (>100,000),and good alignment of carbon nanotubes were believed to make the ceramicnanocomposites of Example 1 demonstrate ultra-high electricalconductivity. The electrical conductivity along (σ_(∥)) and vertical(σ⊥) to the length direction is summarized at FIG. 5A and FIG. 5B.

As the electron charge ran along the length of the carbon nanotube muchmore readily than it jumped through the carbon nanotube, the carbonnanotube alignment was believed to lead to the anisotropy of σ_(∥) andσ⊥ for all of the samples. In this example, the values of σ_(∥) weremuch higher than that of σ⊥, which was also demonstrated by the value ofσ_(∥)/σ⊥.

For the CNTs sheet, the values of σ_(∥) and σ⊥ were 5.3×10⁴ and 1.6×10⁴S·m⁻¹ separately, meaning σ_(∥)/σ⊥ was equal to 3.3. The high electricalconductivity was attributed to the percolation network of carbonnanotubes. The high concentration and alignment was believed to realizea high degree of carbon nanotubes contact. It was believed that therewere two sources of electrical resistance in the CNTs sheet: theintrinsic resistance along the carbon nanotubes and the contactresistance at the carbon nanotube junction points. It was believed thatthe contact resistance strongly depended on the contact length at thecarbon nanotube junctions, carbon nanotube diameter, and atomicstructures in the contact region. The high concentration and alignmentpromised a high degree of contact between rigid neighboring carbonnanotubes.

After the polymeric precursor was infiltrated into the carbon nanotubesheet to make the CNTs/PSZ of Example 1, it was believed that the poresin the aligned carbon nanotube sheet were at least partially filled withpolymeric precursor. The surfaces of the ceramic composites of thisexample were observed using a scanning electron microscope (SEM, JEOLJSM-7401F) and atomic force microscopy (AFM, Veeco Instruments Inc.). Athin polymer layer was coated on the surface of the carbon nanotubesheet. Due to the fact that the polymer was insulating, both σ_(∥) andσ⊥ reduced slightly compared to the value of the CNT sheet, as shown atFIG. 5A and FIG. 5B. The pyrolysis process from CNTs/PSZ to CNTs/SiCN1caused a dramatic change of the electrical conductivity. The value ofσ_(∥) for CNTs/SiCN1 reached up to 2.3×10⁵ S·m⁻¹ (see FIG. 5A). When thematerials of Example 1 were compared with other representative carbonnanotube-ceramic nanocomposites and graphite, it was found that theelectrical conductivity of the composite materials of Example 1 were atthe same level as that of graphite in the basal plane, as shown at FIG.5C. Specifically, the electrical conductivity of the ceramic compositesalong the alignment direction was 2.2×10⁵ S·m⁻¹, which was at the samelevel as the electrical conductivity of graphite in the basal plane. Theanisotropy of electrical conductivity increased from 3.3 to 9.2 afterthe pyrolysis process. Therefore, unprecedentedly high electricalconductivity was surprisingly achieved in the composite materials ofExample 1. These results also were believed to indicate that thecomposite materials of Example may be useful for electromagneticinterference shielding.

As explained at Example 1, the precursor was transferred from a polymerstate to a ceramic state with shrinkage occurring during the pyrolysisprocess. Not wishing to be bound by any particular theory, thisshrinkage was believed to improve the electrical conductivity of theceramic nanocomposites for at least one of two reasons. First, theshrinkage was believed to change the sample size, especially along thewidth and thickness directions. The shrinkage likely caused the carbonnanotubes to stack more closely together, thereby improving electrontransport. Second, as seen in SEM images, the thin polymer layer on thesurface of the carbon nanotube sheet likely hampered the electron chargefor CNTs/PSZ, and the shrinkage likely exposed carbon nanotubes on thesurface for CNTs/SiCN1. Some carbon nanotubes appeared to be exposedoutside. As a result, it was believed that the pyrolysis process playedan important role on the mechanical and electrical properties ofresultant ceramic nanocomposites.

These explanations were verified by the microstructure characterization(via SEM and AFM (i.e., atomic force microscopy)) of the nanocomposites'surface from CNTs/PSZ and CNTs/SiCN1. The value of σ⊥ almost doubledfrom CNTs/PSZ to CNTs/SiCN1, as shown at FIG. 5B, which was believed tobe due to the closer packing of carbon nanotubes. However, σ_(∥) wassignificantly higher than σ⊥ and σ_(∥)/σ⊥ increased to 9.2 after thepyrolysis process, which was believed to demonstrate more significantanisotropy for CNTs/SiCN1. After the PIP process was repeated, asexplained at Example 1, CNTs/SiCN2's electrical conductivity became2.2×10⁵ S·m⁻¹, and the σ_(∥)/σ⊥ was still 9.2.

Example 5—Thermal Properties

The thermal properties of the composite material of Example 1 weretested. The thermal properties were characterized using a laser beam andthe temperature was read from an Infrared camera.

During the experiment, a laser beam was shot at the center of thesample. The temperature profiles at two different locations were tested:[1] a spot in the middle of the sample, and [2] a spot on the edge ofthe sample (along the longitude direction). These tests revealed thatthe ceramic membrane had a relatively high thermal conductivity, whichwas believed to permit heat to be dissipated quickly from the heatingsource to surrounding areas.

The temperature-dependent in-plane thermal conductivity of CNTs/SiCN1 isdepicted at FIG. 6. It was believed that due to the fact that thecomposite material of Example 1 was obtained from an aligned carbonnanotube sheet having a stretch ratio of 35%, the thermal transportalong the alignment direction was improved in the resulting compositematerials. At room temperature, the thermal conductivity increased up to84 W/(m*K) along the alignment direction.

I claim:
 1. A method of forming a composite material, the methodcomprising: providing a carbon nanoscale fiber network which comprises aplurality of substantially aligned carbon nanoscale fibers; infiltratingthe carbon nanoscale fiber network with a first amount of a liquidceramic precursor; curing the first amount of the liquid ceramicprecursor to form a cured ceramic precursor; and pyrolyzing the curedceramic precursor to form the composite material; wherein the compositematerial comprises the carbon nanoscale fibers at a volume fraction ofat least 35%.
 2. The method of claim 1, wherein the volume fraction ofthe carbon nanoscale fibers in the composite material is about 40% toabout 80%.
 3. The method of claim 1, wherein the electrical conductivityof the composite material is about 2.0×10⁴ S/m to about 3.0×10⁴ S/m. 4.The method of claim 1, further comprising: infiltrating a second amountof the liquid ceramic precursor into the composite material; curing thesecond amount of the liquid ceramic precursor to form a second amount ofa cured ceramic precursor; and pyrolyzing the second amount of the curedceramic precursor.
 5. The method of claim 4, wherein the volume fractionof carbon nanoscale fibers in the composite material is about 50% toabout 70%.
 6. The method of claim 4, wherein the electrical conductivityof the composite material is about 2.0×10⁴ S/m to about 2.5×10⁴ S/m. 7.The method of claim 4, wherein the composite material is (i) flexible,and (ii) has a tensile strength of at least 400 MPa.
 8. The method ofclaim 1, wherein the providing of the carbon nanoscale fiber networkcomprises: providing a carbon nanoscale fiber network which comprises aplurality of randomly oriented carbon nanoscale fibers; and stretchingthe carbon nanoscale fiber network to substantially align the pluralityof randomly oriented carbon nanoscale fibers, wherein the stretching ofthe carbon nanoscale fiber network imparts the carbon nanoscale fibernetwork with a stretch ratio of about 10% to about 70%.
 9. The method ofclaim 8, wherein the stretch ratio is about 25% to about 45%.
 10. Themethod of claim 8, wherein the stretch ratio is about 35%.
 11. Themethod of claim 1, wherein the plurality of substantially aligned carbonnanoscale fibers comprises single-wall carbon nanotubes, multi-wallcarbon nanotubes, or a combination thereof.
 12. The method of claim 1,wherein the liquid ceramic precursor is polysilazane.
 13. A compositematerial comprising: a ceramic material dispersed in a carbon nanoscalefiber network which comprises a plurality of substantially alignedcarbon nanoscale fibers, wherein the composite material (i) includes avolume fraction of the carbon nanoscale fibers of at least 35%, and (ii)has an electrical conductivity of at least 1.75×10⁴ S/m.
 14. Thecomposite material of claim 13, wherein the composite material isflexible.
 15. The composite material of claim 13, wherein the volumefraction of carbon nanotubes in the composite material is about 40% toabout 80%.
 16. The composite material of claim 13, wherein theelectrical conductivity of the composite material is about 2.0×10⁴ S/mto about 3.0×10⁴ S/m.
 17. The composite material of claim 13, whereinthe composite material has a tensile strength of at least 400 MPa. 18.The composite material of claim 13, wherein the composite material has atensile strength of about 450 MPa to about 650 MPa.
 19. The compositematerial of claim 13, wherein the ceramic material comprises siliconcarbonitride.
 20. The composite material of claim 13, wherein theceramic material is substantially evenly dispersed in the carbonnanoscale fiber network.
 21. The composite material of claim 13, whereinthe carbon nanoscale fibers comprise single-wall carbon nanotubes,multi-wall carbon nanotubes, or a combination thereof.